Integrated sedimentological, geophysical and geotechnical study of inner shelf sediments in Hong Kong

Abstract

The physical properties of inner continental shelf sediments can be used to provide a record of sequence stratigraphy. Previous work on an offshore borehole located in the West Lamma Channel, Hong Kong, has revealed a sedimentary sequence dating back to ca. 0.5 Ma with five interglacial-glacial cycles consistent with the Vostok ice core in Antarctica. In the present study, two new vibrocores located adjacent to the site of previously rotary borehole found to show five marine transgressive units were examined for their sedimentological, geotechnical and geophysical properties. The results obtained include (1) In-situ cone penetration test (CPT); (2) laboratory measurements of p-wave velocity, magnetic susceptibility, moisture content, density and sedimentological core logging; and (3) automated measurements of properties and collection of digital images using the GEOTEK multi-sensor core logging system with GEOSCAN. The properties of the sea-floor sediments were found to be related to both the depositional and post-depositional processes. Five marine units (M1 to M5) and three terrestrial units (T2 to T4) can be recognized. The Holocene M1 unit has never been sub-aerially exposed, the clayey silt is soft and saturated, shells and shell fragments are preserved and properties including moisture content, porosity, magnetic susceptibility, impedance and density differ from the pre-Holocene M2 to M5 units. Variable properties were found reflecting the more desiccated upper part in comparison to the less desiccated lower part because of subaerial exposure of the M2 to M5 units during glacial period(s). The T2 to T4 units mainly comprising of sand and gravel are different from the M2 to M5 units in their higher cone resistance, density, and p-wave velocity and their lower friction ratio, moisture content, porosity and impedance. The application of fast and non-destructive measurements of physical properties of sediment cores shows great potential for supporting offshore ground investigation in Hong Kong.